Conformational Change of Benzoquinone Crown Ether Radical Anions Induced by Complexation with Alkali Metal Cation: An ESR Study

Abstract

Abstract The crowned p-benzoquinone anion radicals, O5−–O8−, were generated either electrochemically or by using a reducing metal, and their ESR spectra were examined in both the absence and presence of the Na+ or K+ cation. The hyperfine coupling (hfc) constants of the methylene protons bonded to the quinone ring depend largely on the size of the crown ring; the orientation of the benzoquinone skeleton with respect to the crown ring was analyzed. The metal-free O5− has a fixed conformation in which the benzoquinone ring is nearly perpendicular to the crown ring, while O8− interconverts freely between two equivalent conformations. The critical case is O6−; the line-width alternation observed in its ESR spectra was well-analyzed using the two-jump model. Temperature-dependent ESR spectra and MNDO calculations have suggested that O6− undergoes a restricted interconversion between two conformations in which the benzoquinone ring has an orientation similar to that of O5−. An activation energy of 10.0 kcal mol−1 was determined for this motion. The ESR spectra of the ion-paired O5−–O8− anion radicals have suggested that the conformational motion of the anion radicals is fixed by complexation with the Na+ or K+cation. The hfc constants of the methyl protons in the benzoquinone ring increased upon complexation, indirectly confirming the interaction between the carbonyl oxygen of benzoquinone at the crown-ring side and the metal cation.